Doing a obd1 conversion on this gen 3 prelude. We got a P28 ecu that was converted from auto to manual and prepped for datalogging. Burned a non-Vtec basemap with CROME to an SST chip and used a rywire conversion harness.

Everything looked good until ignition on. Heard a pop. Cranked, but no start. Switched back to the obd0 ecu and it started no problem. Opened the P28 and found the C60 capacitor in peices. I am loading photos to photobucket.

I understand this capacitor is related to Vtec, but we aren't using Vtec controls for anything. I wonder if the extra pigtail with the conversion harness was wired right, but I think the only system on the pigtail is our O2 sensor.

Anyway, would this blown capacitor keep the car from starting?

Thanks in advance.

 

Yeah, that's probably not all you fried. C60 doesn't have a direct node to the harness except the ground. The corresponding transistors also may be fried. You should check all the input pins for burn marks as well as the ground nodes. I can almost guarantee your jumper harness is the cause of the short, but if you faulted the ground you'll probably need a new ECU.
You said you plugged in an OBD0 ecu and it ran fine, I hope you're using an OBD1 distributor with a P28...

 

3rd gen preludes have obd1 electronics from the factory, they just still use an obd0 ecu and obd0 single-wire o2 sensors. the conversion harness just arranges the wires to match up with obd1 ecu plugs, and adds external wiring for a single 4-wire heated o2 sensor.
(notice my sig below)
P.S. get a boomslang conversion harness, there is one specifically for 88-91 preludes. the harness you got is probably a standard obd0-obd1 harness, and it does NOT work with 3g preludes.

 

go here and read it, and select from the drop down menu at the bottom:
http://boomslang.us/obd0-to-obd1.php
if you look at the part number for the prelude, you'll notice its different than the part number for all other Hondas/acuras

 

Rywire configured this harness for a B20A5. None of the pins on the harness or ECU look burned. I'll take a closer look this morning with better light. What other components should I test?

I'm pretty useless with photosharing, so forgive me if this doesn't work:








Thanks guys.

 

Next time I'll wire everything up except plugging in the ECU. Then I can test which pins go hot when I turn the ignition on. Anyone know which pins those should be? I'm sure I've got a wiring diagram here somewhere...

 

Are you sure you used the correct polarity on C60?

 

This isn't a P28. It's a P06 which was poorly converted to Vtec with an IAB circuit to simulate a P72 or P13.
Since J10 is jumper-wired for the Vtec conversion this makes a direct node connection from C60 to pin B1, in which case you likely toasted D5 also.
Replace the ECU and THOROUGHLY check your wiring for mistakes.
Also don't buy any more ECUs from this clown, is this a local dealer or an ebay seller?

 

motoxxxman may also have a point there, I can't confirm that but I assume he knows a good amount about it.
But I'd really like to confirm you're using an OBD1 distributor with an OBD1 ECU and that it was professionally repinned or jumpered, because if you shorted pin B1 from improper distributor wiring that would cause this issue also.

 

Yeah, it's from Ebay. I installed the IAB components per recommendation from Xenocron. Regarding the IAB it was dumb to make the change since this whole thing is going to be boosted and the IAB are actuated off manifold vacuum. I could plumb a vacuum line in front of the supercharger I guess...

Since I am not using Vtec, can I remove the J10 jumper, replace the fried components (C60, D5, etc.) and give it another go? What's the worst that could happen?

Thanks for all of your input.

 

It's not actuated by the manifold, it comes from a vacuum tank. If you don't have that then your IABs are engaging backwards.
More to the point if you're using a supercharger then you replaced the manifold, so THERE ARE NO IABs.

99% sure you didn't relocated the MAP sensor to the pressure side of the blower so it's not reading positive pressure. Read the JSRC threads.

If it's not starting then the VTEC circuit is irrelevant, something else is wrong. If you have no circuit knowledge it will be impossible for you to diagnose it.

You CAN send it to Xenocron to repair for a nominal fee if you wish, or you can buy a warrantied ECU from a reputable source (me).

You STILL have not verified whether you're using an OBD1 distributor and what dizzy plug style you're using.

 

88-91 preludes (3rd gen) have obd1 dizzys stock. I said that already. it's not an opinion, its a fact.
although if you want to get technical, we also have a separate cam sensor. the dizzy holds the tdc and ckp sensors on the intake cam, and a separate sensor on the exhaust cam reads cyp. the trigger wheels and their respective pickups for all 3 of these sensors are in fact obd1 though. I've been tuning 3g preludes for almost 10 years now, trust me on this. his car itself is proper, the only thing to be of concern is the conversion harness, because the obd0-plug 3g preludes do require a specific conversion harness made specifically for only 88-91 preludes.

(I said above "obd0-plug 3g preludes" because 90-91 preludes that originally came with a b21a1 actually have obd1 ecu plugs and obd1 o2 sensor)

 

Thanks motoxxxman, I think you are tracking what's going on. I talked with Ryan at Rywire; he doesn't remember making any modifications for my order, so it is likely the conversion harness. I'm going to print the wiring diagram today and check the hot pins tomorrow. Hopefully by then I will hear back from Rywire about re-working their harness so this doesn't happen again.

By the way, this is not an off the shelf supercharger job. The IABs are still in place but figuring out how to actuate them is probably not going to happen in the next 2.5 weeks. It isn't important, they are a "nice to have," not a "must have."

We are never going to use Vtec and I will happily disable it. I just want to get the car running NA on any chipped OBD1 ECU so I can start datalogging. I would prefer to fix this ECU if possible. If all else fails, FMU. Good times.

 

I need to know if converting this f-ed up ECU back to non-Vtec is a dumb idea. It seems like I can remove all of the Vtec conversion components and take this thing back to P06. I'm never using Vtec.

In other news, Ryan from rywire spent half and hour last night talking me through all the pins I can disconnect in my conversion harness. I'll upload pin-outs I got from another guy for his home made OBD0-1 harness when I get home tonight. With this knowledge and advice on the reverse conversion, hopefully I can fix this ECU and get it to work.

motoxxxman, what's the deal with the boomslang harness? how does it differ from a typical conversion harness?

Looking up D5 diode and C60 cap values...

 

It lives! Got it running first try with the Boomslang harness. First, I removed the remnants of C60 and J10 jumper. I also bench tested the D5 transistor. It worked fine as a diode and the overvoltage protection function kicked in at 26vdc. On to the next problem: datalogging.

 

www.pgmfi.org
virtually all datalogging problems have already been resolved there. if it's a new one, link to the thread and I'll help.